A known automatic transmission is automatically shifted to establish a selected one of its gear positions, by selective engaging and releasing actions of clutches and brakes each of which has a plurality of frictional coupling elements. The rotating speed of a rotary member of the clutch may be detected by a rotary speed sensor. For example, the rotary speed sensor is disposed to detect teeth (protrusions) and recesses (grooves) formed on an outer circumferential surface of a clutch drum which has an inner spline formed on its inner circumferential surface to support a plurality of frictional coupling elements in the form of frictional plates. The teeth and recesses defining the outer circumferential surface of the clutch drum correspond to teeth (protrusions) and recesses (grooves) of the inner spline. In this case, the rotating speed of the clutch drum is detected by the rotary speed sensor. JP-10-339368 A discloses an example of a clutch device provided with such a rotary speed sensor.
In one type of a known clutch device, a clutch piston is disposed radially outwardly of a clutch drum such that the clutch piston is axially movable relative to the clutch drum. In the clutch device of this type, the clutch piston has an inner spline which is formed on its inner circumferential surface and which engages an outer spline formed on the outer circumferential surface of the clutch drum. This clutch piston has teeth and recesses on its outer circumferential surface which correspond to the teeth and recess of the inner spline. To detect the rotating speed of the clutch piston, a rotary speed sensor is disposed radially outwardly of the clutch piston, to detect the teeth and recesses on the outer circumferential surface of the clutch piston. The inner spline of the clutch piston functions as means for preventing rotation of the clutch piston relative to the clutch drum.
Where the clutch piston has the inner spline engaging the outer spline of the clutch drum, there is a relatively large resistance to a relative axial sliding movement between the clutch piston and the clutch drum, which may prevent a smooth axial sliding movement of the clutch piston relative to the clutch drum. The axial sliding resistance may be reduced by reducing the number of teeth (the number of protrusions and recesses) of the clutch piston, but the reduction of the number of teeth gives rise to a risk of deterioration of accuracy of detection of the rotating speed of the clutch drum by the rotary speed sensor, in particular, the accuracy of detection when the rotating speed of the clutch drum is relatively low.